I just success repaired my 8751A and it worked fine now.I have checked main PSU,this model and schematic extremely similar to HP8753A/B module but 8751 with one more output line.It's no problem.Next I found post reg(08751-66540),most of output LED flashed once after power-on but no continuous output,I considered some protect signal cause it.I checked this PCB carefully and compare it with HP8753A/B schematic post reg,found they have some common design method,but some are really different.So I tried to find which signal cause protection.I finally found one of them come out from FAN behind mechine, yellow line,if this line rise up to 5V,it means FAN no working.This time FAN really no working.I checked along with FAN power line,and found a fuse resister(1A) on post reg board was burned.I used a fuse to replace it,repower-on,and it worked fine and made very sharp test.
I really appreciated keysight library Le,he helped me find 8751 service manual on library bookbox,even if no scanfiles.
I hope this repair can help people who need help like this.